Implement kqueue(2) support via CONFIG_ELOOP_KQUEUE

Implement kqueue(2) support via CONFIG_ELOOP_KQUEUE

NOTE: kqueue has to be closed and re-build after forking. epoll *should*
do the same, but it seems that wpa_supplicant doesn't need it at least.

I have re-worked a little bit of the epoll code (moved into a similar
kqueue function) so it's trivial to requeue epoll if needed in the
future.

Signed-off-by: Roy Marples <roy@marples.name>